Creature Companion 2016; July: 52, 54.
Anton C. Beynen
Chelates in dog foods
Some dog foods feature chelated trace elements such as zinc, iron, copper, manganese and
selenium. These elements are bound to compounds with multiple carbon atoms and called organic
trace elements. The word chelate has become trendy in the marketplace and works as blanket
term. Technically, chelate structures consist of a ring of atoms with the trace element as link, and
so form a specific class of organically bound trace elements.
Chelated trace elements in petfood are purported to outwork inorganic sources that contain no or
only one carbon atom. It is often claimed that chelates are taken up by intestinal cells more
efficiently, have prominent regulatory and catalytic activity in the body’s metabolism and support
the immune system. A few dog foods highlight added organic zinc as booster of a soft and shiny
Trace elements are essential nutrients. Most commercially prepared canine foods are
supplemented with trace elements and contain higher amounts than dogs’ requirements. As
additives, either inorganic or organic elements can be used. If a certain organic element is
absorbed by the small intestine more efficiently, a smaller element quantity meets the intended
absorbable dose. Nevertheless, the use of chelates is more expensive, but this may not discourage
application as marketing tool.
Downregulation of absorption protects the body against excess of trace elements. The intestinal
uptake routes of chelates might bypass the body’s security mechanism. Little is known about the
nature of degradation products of intactly absorbed chelates. Research data in the public domain
cannot substantiate chelate-mediated health advantages in dogs. All in all, organic trace elements
do not appear to be superior to inorganic sources.
Petfood and trace elements
Fear for deficiencies and process simplification explain why commercial petfoods are supplemented
with liberal amounts of trace elements. Food-specific supplementation is no easy task. For each
element it involves determining the background concentration in the diet, estimating absorbability,
setting the supply of the target animal and judging the need and form of supplementation.
The conceptual relationship between intake of an individual trace element and health status of an
animal spans amounts ranging from deficiency to adequacy to toxicity. Dietary supplementation
appears pointless when supply is in the adequacy range. There is no evidence that higher intakes
improve canine health while homeostatic regulation inhibits absorption and/or stimulates excretion.
Below the adequacy range, supplementation promotes health. Smaller quantities are needed for
trace elements with higher absorbability.
Efficiency of absorption
For dogs, there only is published information on the absorption of organic zinc, but it was not
quantified. Upon ingestion of equal amounts of zinc in gelatin capsules as either zinc oxide or zinc
propionate, the latter induced higher plasma zinc concentrations, indicating more efficient
absorption (1). Similar results were obtained when a zinc-amino acid chelate (2) or zinc proteinate
(3) was compared with zinc oxide.
The zinc content of a dry food without added zinc was almost doubled by incorporating zinc oxide or
zinc-amino acid chelate (4). The chelate lowered fecal zinc excretion. Increasing dietary calcium
raised fecal zinc output for the food with zinc oxide, but not for that with zinc chelate (4). When a
solution with zinc sulfate or zinc methionylglycinate was administered orally after each meal of
extruded food, the apparent absorption percentage of total ingested zinc was 30 and 37% (5).
Organic versus inorganic zinc has enhanced absorbability in dogs. Organic trace elements may be
more soluble in the intestinal content, which increases availability for absorption. They might easily
pass the mucus and unstirred water layer on the intestinal wall. Possibly, chelates cross the mucosa
through passive transport or carrier systems for their organic moiety. These routes would bypass
homeostatic downregulation of absorption which is undesired from a teleological point of view.
Trace elements that are absorbed as organic molecule must be released prior to utilization as co-
factor of enzymes and for other cellular functions. The delivery might be associated with the
formation of undesired substances. High intakes of either sodium selenite or selenomethionine
induced similar hepatic accumulation of selenium in dogs (6), most likely encased in selenocysteine
or selenomethionine along with selenocysteine.
Organic zinc and hair quality
Dogs fed a complete food containing organic instead of inorganic zinc grew more hair of higher zinc
content (4, 5, 7, 8). Methionine (4, 5, 7) or a hydroxyl analogue (8) was a constituent of the organic
zinc source, but the amount added seems negligible. The increased zinc retention in hair was
attributed to higher availability of zinc from the organic sources. However, the observation may
reflect zinc accumulation caused by uncontrolled absorption and disturbed zinc homeostasis.
Organic zinc induced smoother and less fragmented hair as based on electron microscopy (5, 9).
Veterinarians blinded to treatment modality assigned statistically, significantly higher brightness
scores to the hair of supplemented dogs (8). The mean scores were 2.4 and 2.1 on a 1-3 scale which
questions practical significance.
Substantiation of claims
There are no research data in the public domain showing that trace elements in either inorganic or
organic form improve canine health at intakes higher than the recommended allowances. Petfood
manufacturers wishing to use chelates for health claims must rely on the suppliers’ data.
1. Brinkhaus F, Mann J, Zorich C, Greaves JA. Bioavailability of zinc propionate in dogs. J Nutr 1998;
2. Lowe JA, Wiseman J, Cole DJA. Absorption and retention of zinc when administered as an amino-
acid chelate in the dog. J Nutr 1994; 124: 2572S-2574S.
3. Wedekind KJ, Lowry SR. Are organic zinc sources efficacious in puppies? J Nutr 1998; 128: 2593S-
4. Lowe JA, Wiseman J, Cole DJA. Zinc source influences zinc retention in hair and hair growth in the
dog. J Nutr 1994; 124: 2575S-2576S.
5. Jamikorn U, Preedapattarapong T. Comparative effects of zinc methionylglycinate and zinc sulfate
on hair coat characteristics and zinc concentration in plasma, hair, and stool of dogs. Thai J Vet Med
2008; 38: 9-16.
6. Todd SE, Thomas DG, Bosch G, Hendriks WH. Selenium status in adult cats and dogs fed high
levels of dietary inorganic and organic selenium. J Anim Sci 2012; 90: 2549-2555.
7. Lowe JA, Wiseman J. A comparison of the bioavailability of three dietary zinc sources using four
different physiologic parameters in dogs. J Nutr 1988; 128: 2809S-2811S.
8. Trevizan L, Fischer MM, Rodenbush CR, Labres RV, Kessler AM. Effects of diets containing organic
and inorganic zinc sources on hair characteristics, zinc concentration in blood and hair, and the
immune response of dogs. Acta Sci Vet 2013; 41: 1154.
9. Kuhlman G, Rompala RE. The influence of dietary sources of zinc, copper and manganese on
canine reproductive performance and hair mineral content. J Nutr 1998; 128: 2603S-2605S.